Capture verification using an evoked response reference

ABSTRACT

A method and system for verifying capture in the heart involves the use of pacing artifact templates. One or more pacing artifact templates characterizing a post pace artifact signal associated with a particular pace voltage or range of voltages are provided. A pacing artifact template is canceled from a cardiac signal sensed following a pacing pulse. Capture is detected by comparing the pacing artifact canceled cardiac signal to an evoked response reference. Fusion/pseudofusion detection involves determining a correlation between a captured response template and a sensed cardiac signal.

RELATED PATENT DOCUMENTS

This is a division of patent application Ser. No. 10/335,599, filed onDec. 31, 2002, now U.S. Pat. No. 7,191,004 issued on Mar. 13, 2007, towhich Applicant claims priority under 35 U.S.C. §120, and which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to verifying capture of the heart following thedelivery of a pace pulse.

BACKGROUND OF THE INVENTION

When functioning normally, the heart produces rhythmic contractions andis capable of pumping blood throughout the body. However, due to diseaseor injury, the heart rhythm may become irregular resulting in diminishedblood circulation. Arrhythmia is a general term used to describe heartrhythm irregularities arising from a variety of physical conditions anddisease processes. Cardiac rhythm management systems, such asimplantable pacemakers and cardiac defibrillators, have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically comprise circuitry to sense electrical signals from the heartand a pulse generator for delivering electrical stimulation pulses tothe heart. Leads extending into the patient's heart are connected toelectrodes that contact the myocardium for sensing the heart'selectrical signals and for delivering stimulation pulses to the heart inaccordance with various therapies for treating the arrhythmias.

Cardiac rhythm management systems operate to stimulate the heart tissueadjacent to the electrodes to produce a contraction of the tissue.Pacemakers are cardiac rhythm management systems that deliver a seriesof low energy pace pulses timed to assist the heart in producing acontractile rhythm that maintains cardiac pumping efficiency. Pacepulses may be intermittent or continuous, depending on the needs of thepatient. There exist a number of categories of pacemaker devices, withvarious modes for sensing and pacing one or more heart chambers.

When a pace pulse produces a contractile response in heart tissue, thecontractile response is typically referred to as capture, and theelectrical cardiac signal corresponding to capture is denoted the evokedresponse. Superimposed with the evoked response may be a pacing artifactsignal including, for example, the signal associated with post paceresidual polarization. The magnitude of the pacing artifact signal maybe affected by a variety of factors including lead polarization, afterpotential from the pace pulse, lead impedance, patient impedance, pacepulse width, and pace pulse amplitude, for example.

A pace pulse must exceed a minimum energy value, or capture threshold,to produce a contraction. It is desirable for a pace pulse to havesufficient energy to stimulate capture of the heart without expendingenergy significantly in excess of the capture threshold. Thus, accuratedetermination of the capture threshold is required for efficient paceenergy management. If the pace pulse energy is too low, the pace pulsesmay not reliably produce a contractile response in the heart resultingin ineffective pacing. If the pace pulse energy is too high, the resultmay be patient discomfort as well as shorter battery life.

Capture detection allows the cardiac rhythm management system to adjustthe energy level of pace pulses to correspond to the optimum energyexpenditure that reliably produces a contraction. Further, capturedetection allows the cardiac rhythm management system to initiate aback-up pulse at a higher energy level whenever a pace pulse does notproduce a contraction.

A fusion beat is a cardiac contraction that occurs when two intrinsiccardiac depolarizations of a particular chamber, but from separateinitiation sites, merge. When the heart is being paced, a fusion beatmay occur when an intrinsic cardiac depolarization of a particularchamber merges with a pacer output pulse within that chamber. Fusionbeats, as seen on electrocardiographic recordings, exhibit variousmorphologies. The merging depolarizations of a fusion beat do notcontribute evenly to the total depolarization.

Pseudofusion occurs when a pacer output pulse artifact is superimposedupon a spontaneous P wave during atrial pacing, or upon a spontaneousQRS complex during ventricular pacing. In pseudofusion, the pacingstimulus is ineffective because the tissue around the electrode hasalready spontaneously depolarized and is in its refractory period.

During normal pacing, the presence of fusion and pseudofusion beats maybe of little consequence except for wasted energy due to the generationof unnecessary pace pulses. However, detection of fusion andpseudofusion beats may be required during an automatic capture orthreshold determination procedures. Fusion and pseudofusion beats maycause false detection of capture and may lead to erroneous capturethreshold values.

Capture may be verified by detecting if a cardiac signal following apace pulse indicates an evoked response. However, the evoked responsemust be discerned from the superimposed post pace residual polarization,denoted herein as a pacing artifact. In addition, fusion or pseudofusionbeats may further obscure evidence of capture. It is desirable to detectthe evoked response and thereby verify capture so that an effective pacepulse energy may be chosen and appropriate back up pacing delivered. Forthe reasons stated above, and for other reasons stated below which willbecome apparent to those skilled in the art upon reading the presentspecification, there is a need in the art for a method and device thatreliably and accurately detects capture in a patient's heart by sensingan evoked response in the presence of the post pace residualpolarization and possible fusion or pseudofusion beats. There exists afurther need for such an approach that is adaptive and accommodateschanges in the patient's capture threshold over time. The presentinvention fulfills these and other needs.

SUMMARY OF THE INVENTION

The present invention is directed to a method and device for verifyingcapture in the heart by comparing an evoked cardiac response with anevoked response reference. In accordance with one embodiment of thepresent invention, one or more pacing artifact templates are providedthat characterize a pacing artifact signal associated with various pacevoltage levels. An evoked response reference is determined. A cardiacsignal is sensed following a pace pulse and a particular pacing artifacttemplate is canceled from the cardiac signal. Capture is determined bycomparing the pacing artifact canceled cardiac signal and the evokedresponse reference.

Another embodiment of the invention involves a method for detecting afusion/pseudofusion cardiac beat. According to this embodiment, acaptured response template is provided. The fusion/pseudofusion beat isdetected by comparing a sensed cardiac signal to the captured responsetemplate.

Yet another embodiment of the invention involves a medical device fordetecting capture in a patient's heart. The medical device includes alead system extending into the heart including electrodes forstimulating and/or sensing the heart. Pulse generator circuitry iscoupled to the lead system and is configured generate pulses tostimulate the heart. Sensing circuitry is coupled to the lead system andis configured to sense cardiac signals transmitted through the leadelectrodes. A control system controls operation of the device, includingthe pulse generator circuitry and the sensing circuitry. A capturedetection system is coupled to the sensing circuitry and the controlsystem. The capture detection system is configured to provide a pacingartifact template associated with a post pace signal and to determine anevoked response reference indicative of capture of the heart. Thecapture detection system cancels the pacing artifact template fromcardiac signals sensed following a pulse, and detects capture of theheart by comparing the pacing artifact canceled cardiac signals and theevoked response reference.

A further embodiment of the invention involves a medical device fordetection fusion/pseudofusion beats. The medical device includes a leadsystem comprising electrodes and extending into the heart. Pulsegenerator circuitry is coupled to the lead system and is configuredgenerate pulses to stimulate the heart. Sensing circuitry is couple tothe lead system and is configured to sense cardiac signals transmittedthrough the lead electrodes. A control system controls operation of thedevice, including the pulse generator circuitry and the sensingcircuitry. A fusion/pseudofusion detection system and is coupled to thesensing circuitry and the control system. The fusion/pseudofusion systemis configured to determine a captured response template and to detectfusion/pseudofusion beats by comparing the sensed cardiac signals andthe captured response template.

In accordance with another embodiment of the invention, a system fordetecting capture of a patient's heart includes means for providing apacing artifact template, the pacing artifact template characterizing apacing artifact signal. The system further includes means for providingan evoked response reference, the evoked response reference indicativeof capture of the heart, means for canceling the pacing artifacttemplate from a cardiac signal sensed following a pacing pulse, andmeans for detecting capture of the heart by comparing the pacingartifact canceled cardiac signal and the evoked response reference.

In accordance with yet another embodiment of the invention, a system fordetecting a fusion/pseudofusion beat includes means for determining acaptured response template, means for sensing a cardiac signal, andmeans for detecting the fusion/pseudofusion beat by comparing the sensedcardiac signal and the captured response template.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial view of one embodiment of an implantable medicaldevice with an endocardial lead system extending into atrial andventricular chambers of a heart;

FIG. 2 is a block diagram of an implantable medical device with whichcapture verification and fusion/pseudofusion detection of the presentinvention may be implemented;

FIG. 3 is a flowchart of a method of detecting capture in accordancewith an embodiment of the present invention;

FIG. 4 is a flowchart of a method of forming a pacing artifact templatein accordance with an embodiment of the present invention;

FIG. 5 is a flowchart of a method of predicting a pacing artifacttemplate as an exponentially decaying function in accordance with anembodiment of the present invention;

FIG. 6 is a graph comparing a predicted pacing artifact to a pacingartifact template;

FIG. 7 is a flowchart of a method of forming an evoked response templatein accordance with an embodiment of the present invention;

FIGS. 8A and 8B are graphs illustrating normalization of a pacingartifact template in accordance with an embodiment of the invention;

FIG. 9 is a graph illustrating cancellation of a pacing artifacttemplate from a sensed cardiac signal in accordance with an embodimentof the invention;

FIG. 10 is a flowchart of a method of providing a peak-to-peak amplitudereference in accordance with an embodiment of the present invention;

FIGS. 11A and 11B are flowcharts illustrating methods of detectingcapture using an evoked response template in accordance with anembodiment of the present invention;

FIGS. 12A and 12B are flowcharts illustrating methods of detectingcapture using a peak-to-peak amplitude reference in accordance with anembodiment of the present invention;

FIGS. 13A and 13B are graphs of a captured response and a pseudofusionbeat, respectively; and

FIG. 14 is a flowchart of a method of detecting fusion/pseudofusionbeats in accordance with an embodiment of the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings forming a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

The present invention describes methods and systems for verifyingcapture following the application of a pace pulse to the heart. Inaccordance with various aspects of the invention, capture verificationmay be implemented by comparing a cardiac signal following the pacepulse to a template, or other reference, representative of an evokedresponse. Furthermore, fusion and/or pseudofusion detection may also beimplemented using the principles of the invention.

By way of example, the processes of the present invention may be used toenhance capture verification and determination the optimal energy forpacing. Determination of the optimal pacing energy may be implemented,for example, by an automatic capture threshold procedure executed by animplantable cardiac rhythm management system. Additionally, automaticcapture verification may be used to monitor pacing on a beat-by-beatbasis and to control back up pacing when a pace pulse delivered to theheart fails to evoke a contractile response. These and otherapplications may be enhanced by employment of the systems and methods ofthe present invention.

Those skilled in the art will appreciate that reference to a capturethreshold procedure indicates a method of measuring the stimulationthreshold in either an atrium or a ventricle. In such a procedure, thepacemaker, automatically or upon command, initiates a search for thecapture threshold of the chamber. In one example of an automatic capturethreshold procedure, the pacemaker automatically decreases the pulseamplitude in discrete steps until a predetermined number of consecutiveloss-of-capture events occur. At that point, the pacemaker may increasethe stimulation voltage in discrete steps until a predetermined numberof capture events occur to confirm the capture threshold. Variousmethods of implementing capture threshold procedures are known in theart and may be enhanced by the capture detection methods of the presentinvention.

Automatic capture threshold determination is distinguishable fromautomatic capture detection, which is a procedure that occurs on abeat-by-beat basis. Automatic capture detection confirms on abeat-by-beat basis that a delivered pace pulse results in an evokedresponse. When no evoked response is detected following a pace pulse,the pacemaker may deliver a back up safety pulse to ensure consistentpacing. If a predetermined number of pace pulses delivered during normalpacing do not produce an evoked response, the pacemaker may initiate acapture threshold test as described above. The various procedures forimplementing automatic capture detection and/or back up pacing processesmay be enhanced by the capture detection methods described herein.

The embodiments of the present system illustrated herein are generallydescribed as being implemented in an implantable cardiac defibrillator(ICD) that may operate in numerous pacing modes known in the art.Various types of single and multiple chamber implantable cardiacdefibrillators are known in the art and may implement a captureverification methodology of the present invention. Furthermore, thesystems and methods of the present invention may also be implemented avariety of cardiac rhythm management systems, including single and multichamber pacemakers, resynchronizers, and cardioverter/monitor systems,for example.

Although the present system is described in conjunction with animplantable cardiac defibrillator having a microprocessor-basedarchitecture, it will be understood that the implantable cardiacdefibrillator (or other device) may be implemented in any logic-basedintegrated circuit architecture, if desired.

Referring now to FIG. 1 of the drawings, there is shown one embodimentof a cardiac rhythm management system that includes an implantablecardiac defibrillator 100 electrically and physically coupled to anintracardiac lead system 102. The intracardiac lead system 102 isimplanted in a human body with portions of the intracardiac lead system102 inserted into a heart 101. The intracardiac lead system 102 is usedto detect and analyze electrical cardiac signals produced by the heart101 and to provide electrical energy to the heart 101 under certainpredetermined conditions to treat cardiac arrhythmias, including, forexample, ventricular fibrillation of the heart 101.

The intracardiac lead system 102 includes one or more pacing electrodesand one or more intracardiac defibrillation electrodes. In theparticular embodiment shown in FIG. 1, the intracardiac lead system 102includes a ventricular lead system 104 and an atrial lead system 106.The ventricular lead system 104 includes an SVC-coil 116, an RV-coil114, and an RV-tip electrode 112. The RV-coil 114, which mayalternatively be an RV-ring electrode, is spaced apart from the RV-tipelectrode 112, which is a pacing electrode. In one embodiment, theventricular lead system 104 is configured as an integrated bipolarpace/shock lead. In another exemplary configuration, one or moreadditional electrodes, e.g., a ring electrode, may be included in theventricular lead system 104. The additional ring electrode and theRV-tip electrode 112 may be used for bipolar sensing of cardiac signals.The atrial lead system 106 includes an A-tip electrode 152 and an A-ringelectrode 154. In one embodiment, the atrial lead system 106 isconfigured as an atrial J lead.

In this configuration, the intracardiac lead system 102 is positionedwithin the heart 101, with portions of the atrial lead system 106extending into the right atrium 120 and portions of the ventricular leadsystem 104 extending into the right atrium 120 and right ventricle 118.In particular, the A-tip electrode 152 and A-ring electrode 154 arepositioned at appropriate locations within the right atrium 120. TheRV-tip electrode 112 and RV-coil 114 electrodes are positioned atappropriate locations within the right ventricle 118. The SVC-coil 116is positioned at an appropriate location within the right atrium chamber120 of the heart 101 or a major vein leading to the right atrium chamber120 of the heart 101. The RV-coil 114 and SVC-coil 116 depicted in FIG.1 are defibrillation electrodes.

Additional pacing and defibrillation electrodes may also be included inthe intracardiac lead system 102 to allow for various bipolar sensing,pacing, and defibrillation capabilities. For example, the intracardiaclead system 102 may include endocardial pacing andcardioversion/defibrillation leads (not shown) that are advanced intothe coronary sinus and coronary veins to locate the distal electrode(s)adjacent to the left ventricle or the left atrium. Other intracardiaclead and electrode arrangements and configurations known in the art arealso possible and considered to be within the scope of the presentsystem.

The ventricular and atrial lead systems 104, 106 include conductors forcommunicating sense, pacing, and defibrillation/cardioverter signalsbetween the cardiac defibrillator 100 and the electrodes and coils ofthe lead systems 104, 106. As is shown in FIG. 1, ventricular leadsystem 104 includes a conductor 108 for transmitting sense and pacingsignals between the RV-tip electrode 112 and an RV-tip terminal 202 ofthe cardiac defibrillator 100. A conductor 110 of the ventricular leadsystem 104 transmits sense signals between the RV-coil or ring electrode114 and an RV-coil terminal 204 of the cardiac defibrillator 100. Theventricular lead system 104 also includes conductor 122 for transmittingsense and defibrillation signals between terminal 206 of the cardiacdefibrillator 100 and SVC-coil 116. The atrial lead system 106 includesconductors 132, 134 for transmitting sense and pacing signals betweenterminals 212, 210 of the cardiac defibrillator 100 and A-tip and A-ringelectrodes 152 and 154, respectively.

Referring now to FIG. 2, there is shown an embodiment of a cardiacdefibrillator 200 suitable for implementing a capture verificationmethodology of the present invention. FIG. 2 shows a cardiacdefibrillator divided into functional blocks. It is understood by thoseskilled in the art that there exist many possible configurations inwhich these functional blocks can be arranged. The example depicted inFIG. 2 is one possible functional arrangement. The cardiac defibrillator200 includes circuitry for receiving cardiac signals from a heart (notshown in FIG. 2) and delivering electrical stimulation energy to theheart. The cardiac defibrillator 200 includes terminals for connectingthe cardiac defibrillator 200 to the electrodes of the intracardiac leadsystem as previously discussed.

In one embodiment, the cardiac defibrillator circuitry 203 of thecardiac defibrillator 200 is encased and hermetically sealed in ahousing 201 suitable for implanting in a human body as is known in theart. Power to the cardiac defibrillator 200 is supplied by anelectrochemical battery 233 that is housed within the cardiacdefibrillator 200. A connector block (not shown) is additionallyattached to the housing 201 of the cardiac defibrillator 200 to allowfor the physical and electrical attachment of the intracardiac leadsystem conductors to the cardiac defibrillator 200 and the encasedcardiac defibrillator circuitry 203.

The cardiac defibrillator circuitry 203 of the cardiac defibrillator 200may be a programmable microprocessor-based system, including a controlsystem 220 and a memory circuit 236. The memory circuit 236 storesparameters for various pacing, defibrillation, and sensing modes, andstores data indicative of cardiac signals received by other componentsof the cardiac defibrillator circuitry 203. The control system 220 andmemory circuit 236 cooperate with other components of the cardiacdefibrillator circuitry 203 to perform operations involving the captureverification according to the principles of the present invention, inaddition to other sensing, pacing and defibrillation functions. Thecontrol system 220 may encompass additional functional componentsincluding a pacemaker 222, an arrhythmia detector 240 and templategenerator 241 along with other functions for controlling the cardiacdefibrillator circuitry 203. A memory 232 is also provided for storinghistorical EGM and therapy data. The historical data may be used forvarious purposes to control the operations of the cardiac defibrillator200 and may also be transmitted to an external programmer unit 234 asneeded or desired.

Telemetry circuitry 231 is additionally coupled to the cardiacdefibrillator circuitry 203 to allow the cardiac defibrillator 200 tocommunicate with an external programmer unit 234. In one embodiment, thetelemetry circuitry 231 and the programmer unit 234 use a wire loopantenna and a radio frequency telemetric link, as is known in the art,to receive and transmit signals and data between the programmer unit 234and the telemetry circuitry 231. In this manner, programming commandsmay be transferred to the control system 220 of the cardiacdefibrillator 200 from the programmer unit 234 during and after implant.In addition, stored cardiac data pertaining to capture verification andcapture threshold, along with other data, may be transferred to theprogrammer unit 234 from the cardiac defibrillator 200, for example.

Cardiac signals sensed through use of the RV-tip electrode 112 arenear-field signals or rate channel signals as are known in the art. Moreparticularly, a rate channel signal is detected as a voltage developedbetween the RV-tip electrode 112 and the RV-coil 114. Cardiac signalssensed through use of one or both of the defibrillation coils orelectrodes 114, 116 are far-field signals, also referred to asmorphology or shock channel signals, as are known in the art. Moreparticularly, a shock channel signal is detected as a voltage developedbetween the RV-coil 114 and the SVC-coil 116. A shock channel signal mayalso be detected as a voltage developed between the RV-coil 114 and thecan electrode 209. Alternatively, the can electrode 209 and the SVC-coilelectrode 116 may be shorted and a shock channel signal sensed as thevoltage developed between the RV-coil 114 and the can electrode209/SVC-coil 116 combination. Shock channel signals developed usingappropriate combinations of the RV-coil, SVC-coil, and can electrodes114, 116 and 209 are sensed and amplified by a shock EGM amplifier 229.The output of the EGM amplifier 229 is coupled to the control system220.

In the embodiment of the cardiac defibrillator 200 depicted in FIG. 2,RV-tip and RV-coil electrodes 112, 114 are shown coupled to a V-senseamplifier 226 and thus to an R-wave detector 223. Rate channel signalsreceived by the V-sense amplifier 226 are communicated to the R-wavedetector 223, which serves to sense and amplify the rate channelsignals, e.g. R-waves. The sensed R-waves may then be communicated tothe control system 220.

A-tip and A-ring electrodes 152,154 are shown coupled to an A-senseamplifier 225. Atrial sense signals received by the A-sense amplifier225 are communicated to an A-wave detector 221, which serves to senseand amplify the A-wave signals. The atrial signals may be communicatedfrom the A-wave detector 221 to the control system 220.

The pacemaker 222 communicates pacing signals to the RV-tip and A-tipelectrodes 112 and 152 according to a preestablished pacing regimenunder appropriate conditions. Blanking circuitry (not shown) is employedin a known manner when a ventricular or atrial pacing pulse isdelivered, such that the ventricular channel, atrial channel, and shockchannel are properly blanked at the appropriate time and for theappropriate duration.

A switching matrix 228, according to one may be coupled to the A ring154, RV tip 112, RV coil 114 and SVC coil 116 electrodes. The switchingmatrix 228 can be arranged to provide connections to variousconfigurations of pacing and defibrillation electrodes. The outputs ofthe switching matrix 228 are coupled to an evoked response (ER)amplifier 227 which serves to sense and amplify signals detected betweenthe selected combinations of electrodes. The detected signals arecoupled through the ER amplifier 227 to a capture detector 224. Thecapture detector 224 includes circuitry configured to detect an evokedresponse, detect fusion/pseudofusion, and verify capture in accordancewith the invention.

The cardiac defibrillator 200 depicted in FIG. 2 is well-suited forimplementing a capture detection methodology according to the principlesof the present invention. In the embodiment illustrated in FIG. 2, thecapture verification processes of the present invention are primarilycarried out by the capture detector 224 in cooperation with the templategenerator 241 and other components of the control system 220.

The shock channel and rate channel signals used for template operationsin connection with capture verification may be provided by the shock EGMamplifier 229 and the V-sense amplifier 226, respectively. It isunderstood that the required shock and rate channel signals may bedeveloped and processed by components other than those depicted in FIG.2 for system architectures that differ from the system architecturesdescribed herein.

A cardiac signal representing a captured beat may be described in termsof a pacing artifact superimposed on an evoked response. The evokedresponse represents the portion of the cardiac signal generatedprimarily by the contractile response of the heart to the pacing pulse.When a pace pulse does not produce a contractile response, capture doesnot occur, and an evoked response signal is not produced by the heart.The pacing artifact represents the portion of the cardiac signal arisingfrom signal components other than the evoked response, and is typicallysignificantly larger than the evoked response. The pacing artifact maybe affected, for example, by lead polarization, after pace potential,lead impedance, patient impedance, pacer pulse width, and rechargetiming.

The evoked response may be discerned by canceling the pacing artifactfrom a cardiac signal associated with a captured response. Methods forusing the pacing artifact canceled signal to determine capture aredescribed in commonly owned U.S. patent application Ser. No. 10/335,534,now U.S. Pat. No. 7,162,301, which is hereby incorporated herein byreference in its entirety.

According to the principles of the invention, capture verificationincludes using a pacing artifact captured cardiac signal and an evokedresponse reference. The evoked response reference is determined usingthe pacing artifact template. FIG. 3 is a flowchart illustrating variousprocesses for capture verification according to the principles of thepresent invention. According to an example embodiment of the inventionillustrated in the flowchart of FIG. 3, a pacing artifact templaterepresentative of the pacing artifact component of a cardiac signal isprovided 310. An evoked response reference indicative of the evokedresponse component of a captured beat, is also provided 320. The evokedresponse reference is determined using the pacing artifact template. Acardiac signal is sensed 330 following a pace pulse. The pacing artifacttemplate is canceled from the sensed cardiac signal 340. Capture isdetermined by comparing 350 the resultant pacing artifact canceledcardiac signal to the evoked response reference.

FIG. 4 illustrates a method of generating a pacing artifact templateaccording to an embodiment of the present invention. In the embodimentillustrated by FIG. 4, an initial pacing artifact template may be formedby delivering a pulse at a low voltage or during a myocardial refractoryperiod. Either method may be used to generate a pacing artifact waveformwithout a superimposed evoked response. The pacing artifact waveform maybe sensed in a capture verification window and stored as an initialpacing artifact template. Additional pacing artifact waveforms may becombined with the initial pacing artifact template by averaging thepacing artifact waveform with the pacing artifact template, for example.

The additional pacing artifact waveforms used to update the initialtemplate may be produced by pulses delivered 410 to the heart in such away that capture does not occur. Thus, each pulse results in a purepacing artifact waveform without a superimposed evoked response. Purepacing artifact waveforms may be produced, for example, by pace pulsesdelivered 411 to the heart at an energy level lower than the capturethreshold. In one embodiment, the additional pacing artifact waveformsdelivered are pacing artifact waveforms generated by the last 10 pacepulses of a capture threshold stepdown test. Alternatively, theadditional pacing artifact waveforms may be generated by pulsesdelivered 412 during a myocardial refractory period during a time thepulses cannot produce an evoked response.

Following generation of a pace pulse 410 by either of the methodsdiscussed above, the pacing artifact waveform is sensed 420 in thecardiac verification window. The sensed pacing artifact waveforms may becombined with the pacing artifact template, for example, by averaging430 the pacing artifact waveforms with the pacing artifact templatesample by sample. When a predetermined number of pacing artifactwaveforms have been collected 440, pacing artifact template generationis complete 450 and the pacing artifact template may be stored.

The pacing artifact may exhibit small variations in morphology withrespect to pace pulse amplitude. Accordingly, the use of multiple pacingartifact templates corresponding to various pace pulse amplitudes mayprovide a more thorough cancellation of the pacing artifact over a rangeof pace pulse amplitudes, e.g., as used in a pacing threshold test. Themethod illustrated in FIG. 4 can be applied to generate pacing artifacttemplates for each pacing pulse amplitude of interest.

Alternatively, or additionally, a set of two or more pacing artifacttemplates may be generated, wherein a particular pacing artifacttemplate characterizes the pacing artifact associated with a small rangeof pace pulse amplitudes. A pacing artifact template for a pace pulserange can be formed by combining pacing artifact waveforms from variouspace pulse amplitudes within the range using, for example, an averagingoperation. The pacing artifact template for a pace pulse range may alsobe formed by selecting a pacing artifact waveform at a single pace pulseamplitude, e.g., a pacing artifact waveform for a pulse amplitude nearthe center of the range to be characterized. The set of pacing artifacttemplates correspond to the entire pace pulse amplitude range to beevaluated.

The artifact waveform measurement may be accomplished during therefractory period of the myocardium. Pace pulses delivered during therefractory period produce pacing artifact waveforms without the evokedresponse components. The timing of the pace pulse delivered for pacingartifact measurement in the myocardial refractory period should beselected to be before the vulnerable period of the myocardium to avoidpro-arrhythmia, and after the deflections from the myocardial responsefrom the previous cardiac event in the chamber have passed, e.g., 80 msafter the preceding cardiac event.

A pacing artifact may have a shape within the capture verificationwindow that may be characterized by a function. The shape of the pacingartifact may be characterized by an exponentially decaying function inthe capture verification window, for example. According to an exampleembodiment, a number of pacing artifact waveforms sensed during acapture verification window may be combined to form a pacing artifacttemplate. A time constant of the pacing artifact template is determinedand the pacing artifact template is predicted using the estimated timeconstant.

Although the examples provided herein predict the pacing artifact usingan exponential function, those skilled in the art will recognize thatprediction of the pacing artifact is not limited to characterization byan exponential function. Any function or combination of functions may beused to characterize and predict the pacing artifact.

In the exemplary embodiment illustrated by FIG. 5, a pacing artifacttemplate is determined by delivering 510 a predetermined number ofpulses to the heart and sensing 520 the resultant pacing artifactwaveforms in a capture verification window. The pulses are delivered 510in such a way that capture does not occur, resulting in the productionof pure pacing artifact waveforms without a superimposed evokedresponse. The pulses may be delivered at an energy level below thecapture threshold 511, for example. Alternatively, the pace pulses maybe delivered during a myocardial refractory period at a time when pulsesdelivered to the heart do not produce an evoked response 512.

Following delivery 510 of a pulse by either method, the resultant pacingartifact waveform is sensed 520 in the capture verification window. Thesensed pacing artifact waveform is combined 530 with the previouslyacquired waveforms. For example, the pacing artifact waveform may beaveraged with previously acquired pacing artifact waveforms, if any.

The process of delivering a pulse and detecting the resultant pacingartifact waveform 510-530 may be repeated until a predetermined numberof pacing artifact waveforms has been acquired 540. After thepredetermined number of pacing artifact waveforms has been acquired 540,a time constant of the average pacing artifact waveform is estimated550. The pacing artifact template is predicted as an exponentiallydecaying function 560 in the capture verification window using theestimated time constant.

A pacing artifact template generated by either of the methods describedin the preceding paragraphs with reference to FIGS. 4 and 5 may beperiodically updated as required or desired. For example, a pacingartifact template may be updated by averaging additional pacing artifactwaveforms with the existing pacing artifact template.

FIG. 6 shows a comparison of a graph of a pacing artifact template 610representing the average of a number of pacing artifact waveforms and agraph of the pacing artifact template predicted as an exponentiallydecaying function 620. The pacing artifact template can be predicted fora sampled signal using Equation (1):x(t)=A*x(t−1)  (1)where A=e^(−T/a)where x(t) represents a current sample of the pacing artifact template,x(t−1) represents a previous sample of the pacing artifact template, andA is a constant derived from the estimated time constant of the pacingartifact template, a, and the sample time, T.

According to an embodiment of the invention, a signal corresponding tothe pacing artifact template may be generated by hardware using digitalcircuitry to produce a signal corresponding to the function of Equation(1). The normalized pacing artifact template signal generated inhardware may be used to cancel the pacing artifact from a sensed cardiacsignal in the capture verification window. In other embodiments, thenormalized pacing artifact template may be canceled from the sensedcardiac signal using software-based techniques. For example, the pacingartifact template may be canceled by subtracting stored or predictedvalues of the pacing artifact template from the sensed cardiac signal ateach sample point in the capture verification window.

In accordance with various embodiments of the invention, a cardiacsignal representing an evoked response may be characterized by an evokedresponse reference. The evoked response reference characterizes only theevoked response component of a captured response, without a superimposedpacing artifact. The evoked response reference may be used to detectcapture.

A cardiac signal following a pace pulse is sensed and the pacingartifact template is canceled from the cardiac signal. This procedureremoves the pacing artifact from the cardiac signal, leaving only theevoked response portion of the cardiac signal, if capture occurred. Thepacing artifact canceled cardiac signal is compared to an evokedresponse reference. The evoked response reference may be, for example,an evoked response template, an amplitude reference, or other indicatorsassociated with an evoked response. Capture verification is implementedby comparing the evoked response reference to the pacing artifactcanceled cardiac signal following a pace pulse. If a sufficient numberof paced cardiac responses are comparable to the evoked responsereference, then capture may be established.

FIG. 7 illustrates a method of providing an evoked response template foruse in capture verification in accordance with an embodiment of theinvention. The heart is stimulated by pace pulses having a voltagegreater than the capture threshold. The resultant captured cardiacresponses are sensed and averaged. A pacing artifact template issubtracted or otherwise cancelled from the average captured response toproduce an evoked response template.

Turning to a flowchart of this method illustrated in FIG. 7, a pacingartifact template is provided at block 705. For example, the pacingartifact template may be provided by either of the methods discussed inconnection with FIGS. 4 or 5. An average captured response may bedetermined by delivering a predetermined number of pace pulses at apacing voltage greater than the capture threshold 710. Delivery of apace pulse above the capture threshold produces a cardiac contraction.The captured response waveform resulting from the cardiac contraction issensed 720 in the capture verification window. The sensed capturedresponse waveform may be combined 740, for example, by averaging, withthe previously acquired captured response waveforms.

Additional captured response signals may be generated 710 until apredetermined number of captured response waveforms have been acquired750. The average captured response may be stored 760 as a capturedresponse template.

The pacing artifact template is normalized 770 with respect to thecaptured response template in the capture verification window. Followingnormalization, the pacing artifact template is canceled from thecaptured response template 780. For example, the pacing artifacttemplate may be canceled by subtracting the pacing artifact templatefrom the captured response template sample by sample. Subtraction of thepacing artifact template from a captured response template results in anevoked response template 790, which may be stored for use in subsequentcapture verification procedures. The evoked response template may becompared to pacing artifact cancelled cardiac signals to verify capture.

In another embodiment, the pacing artifact template may be normalizedand canceled from a number of captured response beats. The pacingartifact template canceled beats may then be averaged to produce theevoked response template.

Normalization of the pacing artifact template with respect to a cardiacsignal is illustrated in FIGS. 8A and 8B. FIG. 8A shows a graph of apacing artifact template before normalization 810 and afternormalization 820 using one or more values of a sensed cardiac signal830 in the capture verification window. FIG. 8B shows the graph of anormalized pacing artifact template 840 overlaying the graph of a sensedcardiac signal 830 that includes an evoked response.

Cancellation of a pacing artifact template from a sensed cardiac signalin accordance with the invention is illustrated in FIG. 9. A captureverification window commences approximately 25 ms following delivery ofthe pacing pulse and extends for approximately 50 ms. The pacingartifact template 920 is shown overlaying the sensed cardiac waveform ofa captured beat 930. Cancellation of the pacing artifact template 920from the sensed cardiac waveform 930 results in a pacing artifactcanceled waveform 940 from which capture is determined. In the exampleof FIG. 9, capture is indicated by the presence of the local minima 950at approximately 53 ms following the pace pulse.

Capture may be verified by comparing the pacing artifact canceledcardiac signal with the evoked response template. In one implementation,the comparison may be accomplished by calculating a correlationcoefficient (CC) of pacing artifact canceled cardiac signal and theevoked response template by a technique such as Correlation WaveformAnalysis (CWA). Using this technique, correlation coefficient (CC) maybe calculated to compare the pacing artifact canceled cardiac signal tothe evoked response template. In one particular embodiment, Equation 2,provided below, is used to compute the CC between the samples of apacing artifact cancelled cardiac beat and the evoked response templatesamples.

$\begin{matrix}{{CC} = \frac{{N{\sum\limits_{i = 1}^{N}{X_{i}Y_{i}}}} - {( {\sum\limits_{i = 1}^{N}X_{i}} )( {\sum\limits_{i = 1}^{N}Y_{i}} )}}{\sqrt{( {{N{\sum\limits_{i = 1}^{N}X_{i}^{2}}} - ( {\sum\limits_{i = 1}^{N}X_{i}} )^{2}} )( {{N{\sum\limits_{i = 1}^{N}Y_{i}^{2}}} - ( {\sum\limits_{i = 1}^{N}Y_{i}} )^{2}} )}}} & \lbrack 2\rbrack\end{matrix}$where, Xi represents template N samples and Yi represents beat N samplesin this illustrative example. Typically, the number of samplesassociated with each waveform or template is about 33 samples.

If the correlation coefficient is greater than a predetermined value,for example, about 0.71, the pacing artifact canceled cardiac signal isconsidered to represent an evoked response signal. Capture may beestablished when a predetermined number of cardiac responses followingpace pulses of a particular energy level are correlated to the evokedresponse template. In one implementation, capture is established whenthree or more or the last four paced responses are correlated to thetemplate.

The evoked response waveform may change gradually with time. Thissituation may lead to erroneous capture verification because the evokedresponse template no longer represents an evoked response. Updating theevoked response template with evoked response waveforms correlated tothe evoked response template mitigates this problem. The evoked responsetemplate may be updated using evoked response waveforms correlated tothe evoked response template.

In another embodiment of the invention, a peak-to-peak amplitudeindicative of an evoked response may be used as an evoked responsereference. FIG. 10 illustrates the method of providing a peak-to-peakevoked response amplitude reference. A pacing artifact template isprovided 1010, by either of the methods previously discussed inconnection with FIG. 4 and FIG. 5. A pace pulse at a pacing voltagesignificantly above the pacing threshold, for example 4 volts, isdelivered 1020 to the heart and the resulting cardiac signal is sensed1030. The pacing artifact template is normalized with respect to thesensed cardiac signal 1040 in the cardiac verification window. Cancelingthe pacing artifact template from the sensed cardiac signal 1050 resultsin a signal associated with an evoked response. The peak-to-peakamplitude of the evoked response is measured 1060 and stored as theinitial peak-to-peak amplitude reference 1070. The initial peak-to-peakamplitude reference may be subsequently updated by acquiring additionalevoked responses and combining the peak-to-peak amplitudes of theadditional evoked responses with the initial peak-to-peak amplitudereference.

According to methods of the invention, capture may be detected bycomparing an evoked response reference to a pacing artifact canceledcardiac signal. The evoked response reference may be, for example, anevoked response template, a peak-to-peak amplitude reference, or anyother reference or indicator associated with a captured cardiacresponse. The evoked response reference is representative of the cardiacsignal of a captured beat.

FIG. 11A illustrates a method of capture detection using an evokedresponse template for automatic capture verification in accordance withan embodiment of the invention. According to this exemplary embodiment,a pacing artifact template and an evoked response template are provided1101 by methods previously discussed. A pace pulse is delivered 1105 andthe cardiac signal following the pace pulse is sensed 1110. The pacingartifact template is normalized 1120 using one or more samples of thecardiac signal within the capture verification window. In one example,one or more samples of the cardiac signal are averaged and the pacingartifact template normalized with respect to the average value. Inanother example, a representative set of the cardiac signal samples maybe used to define a slope of the cardiac signal within the cardiacverification window. The pacing artifact template may be normalized withrespect to a point extrapolated using the slope.

The pacing artifact template is canceled from the sensed cardiac signal1125, for example, by subtracting the pacing artifact template from thecardiac signal sample by sample, to produce a pacing artifact canceledcardiac signal. If the pacing artifact canceled cardiac signal iscorrelated to the evoked response template, the pacing artifact canceledcardiac signal is detected as an evoked response

When a predetermined number of consecutive beats have pacing artifactcanceled cardiac signals uncorrelated to the evoked response template1150, for example 2 or more beats out of the last 4 paced beats,non-capture is detected 1160. When noncapture is detected 1160, anautomatic threshold test may be initiated 1165 to determine anappropriate pacing voltage. If capture is detected 1170, the processcontinues with the next paced beat 1105. A captured beat may be used toupdate the evoked response reference using, for example, a methoddescribed below in connection with FIG. 11B.

FIG. 11B illustrates a method of capture detection using an evokedresponse template for an automatic threshold process in accordance withan embodiment of the invention. According to this exemplary embodiment,a pacing artifact template and an evoked response template are providedby methods previously discussed.

A pace pulse is delivered 1105 and the cardiac signal following the pacepulse is sensed 1110. The pacing artifact template is normalized 1120using one or more samples of the cardiac signal within the captureverification window. The pacing artifact template may be canceled 1125from the sensed cardiac signal, for example, by subtracting the pacingartifact template from the cardiac signal sample by sample, to produce apacing artifact canceled cardiac signal.

If the pacing artifact canceled cardiac signal is correlated 1130 to theevoked response template, the pacing artifact canceled cardiac signal isdetected as an evoked response and may be used to update 1140 the evokedresponse template. In one example embodiment, a pacing artifact canceledcardiac signal is correlated to the evoked response template when CC isgreater than a predetermined number, for example, 0.71.

If the pacing artifact canceled cardiac signal is correlated to theevoked response template 1130, the evoked response template may beupdated 1140 with the pacing artifact canceled cardiac signal. Theevoked response template may be updated using various techniques, e.g.,using a weighted and/or moving average, or other processes. In oneembodiment, Equation 3, provided below, is used to update each sample ofthe evoked response template with a temporally corresponding sample ofthe pacing artifact canceled cardiac signal in the cardiac verificationwindow:U _(n) =B ₁ C _(n) +B ₂ S _(n)  (3)

where U_(n) represents the n^(th) sample of the updated template, C_(n)represents the n^(th) sample of the current template, S_(n) representsthe n^(th) sample of the pacing artifact canceled cardiac signalcorrelated to the evoked response template, and B₁ and B₂ areappropriate coefficients. In one example, B₁ and B₂ are set equal to0.75 and 0.25, respectively. When a predetermined number of consecutivebeats have pacing artifact canceled cardiac signals uncorrelated to theevoked response template 1150, for example 2 or more beats out of thelast 4 paced beats, non-capture is detected 1160. An appropriate pacingvoltage is established using a suitable safety margin 1165. If captureis detected 1170, the pacing voltage for the threshold test is decreased1175 and the threshold determination process of blocks 1105-1150continues until noncapture is detected.

FIG. 12A illustrates a method of detecting capture using a peak-to-peakamplitude reference during an automatic capture verification process inaccordance with an embodiment of the invention. A pacing artifacttemplate and a peak-to-peak amplitude reference are provided 1210. Thepacing artifact template may be provided, for example, by either of themethods discussed in connection with FIGS. 4 and 5. An initialpeak-to-peak amplitude reference may be provided by the method discussedin connection with FIG. 10. A pace pulse is delivered 1215 and a cardiacsignal following the pace pulse is sensed 1220. The pacing artifacttemplate is normalized within the capture verification window withrespect to the sensed cardiac signal 1225 and canceled from the sensedcardiac signal 1230 to derive the pacing artifact canceled cardiacsignal. The peak-to-peak amplitude of the pacing artifact canceledcardiac signal is measured 1235 and compared to the peak-to-peakamplitude reference 1237. If the peak-to-peak amplitude of the pacingartifact canceled cardiac signal is less than or equal to apredetermined percentage of the peak-to-peak amplitude reference, forexample, 50%, the cardiac signal is classified as a noncaptured beat.Otherwise, the cardiac signal is classified as a captured beat. Thepeak-to-peak amplitude of the captured beat may be used to update thepeak-to-peak amplitude reference using, for example, a method describedbelow in connection with FIG. 12B.

Noncapture may be detected 1270, for example, when a first predeterminednumber out of a second predetermined number of cardiac signals havepeak-to-peak amplitudes less than or equal to than a predeterminedpercentage of the peak-to-peak amplitude reference 1260. Otherwise,capture is detected 1265. For example, noncapture may be detected 1270when 2 or more out of the last 4 paced beats have pacing artifactcanceled cardiac signals with peak-to-peak amplitudes less than or equalto 50% of the peak-to-peak amplitude reference.

If capture is detected 1265, normal pacing continues and another cardiacsignal is sensed 1220. However, if noncapture is detected 1270, anautomatic threshold test may be initiated 1280 to determine anappropriate pacing energy level.

FIG. 12B illustrates a method of detecting capture using a peak-to-peakamplitude reference during a capture threshold test in accordance withan embodiment of the invention. A pacing artifact template and apeak-to-peak amplitude reference are provided, and the initial pacingvoltage is set to a maximum value 1210. The pacing artifact template maybe provided, for example, by either of the methods discussed inconnection with FIGS. 4 and 5. An initial peak-to-peak amplitudereference may be provided by the method discussed in connection withFIG. 10.

A pace pulse is delivered 1215 and a cardiac signal following the pacepulse is sensed 1220. The pacing artifact template is normalized withinthe capture verification window with respect to the sensed cardiacsignal 1225 and canceled from the sensed cardiac signal 1230 to derivethe pacing artifact canceled cardiac signal. The peak-to-peak amplitudeof the pacing artifact canceled cardiac signal is measured 1235 andcompared to the peak-to-peak amplitude reference 1237. If thepeak-to-peak amplitude of the pacing artifact canceled cardiac signal isless than or equal to a predetermined percentage of the peak-to-peakamplitude reference, for example, 50%, the cardiac signal is classifiedas a noncaptured beat. Otherwise, the beat is classified as a capturedbeat.

If a captured beat is detected 1240, the peak-to-peak amplitude of thepacing artifact canceled cardiac signal may be used to update thepeak-to-peak amplitude reference 1245. The peak-to-peak amplitudereference may be updated using various techniques, e.g., using aweighted and/or moving average, or other processes. In one embodiment,the peak-to-peak amplitude of the pacing artifact canceled cardiacsignal is used to update the peak-to-peak amplitude reference accordingto Equation 4 below:U=B ₁ C+B ₂ S  (4)where U represents the updated peak-to-peak amplitude reference, Crepresents the current peak-to-peak amplitude reference, S representsthe peak-to-peak amplitude of the pacing artifact canceled cardiacsignal, and B₁ and B₂ are appropriate coefficients. In one example, B₁and B₂ are set equal to 0.75 and 0.25, respectively.

It may be appropriate to provide an upper and lower bound to thepeak-to-peak amplitude reference as described in Equation 4 above. Forexample, for a ventricular chamber, the updated peak-to-peak amplitudereference may be constrained by an upper bound of 20 mV and a lowerbound of 1 mV. Similarly, for an atrial chamber, the updatedpeak-to-peak amplitude reference may be constrained by an upper bound of4 mV and a lower bound of 0.2 mV.

Noncapture may be detected, for example, when a predetermined number ofcardiac signals have peak-to-peak amplitudes less than or equal to apredetermined percentage of the peak-to-peak amplitude reference 1260.Otherwise, capture is detected 1265. For example, noncapture may bedetected 1270 when 2 or more out of the last 4 paced beats have pacingartifact canceled cardiac signals with peak-to-peak amplitudes less thanor equal to 50% of the peak-to-peak amplitude reference. When noncaptureis detected, the normal pacing voltage is established 1285 using asuitable safety margin. If capture is detected 1265, the pacing voltageis decreased by an incremental value and the threshold test continuesuntil noncapture is detected.

As previously discussed, a situation known as fusion or pseudofusion mayoccur during pacing. A fusion beat occurs when an intrinsic cardiacdepolarization of a particular chamber merges with a pacer output pulsewithin that chamber. Pseudofusion occurs when a pacer output pulseartifact is superimposed upon a spontaneous P wave during atrial pacing,or upon a spontaneous QRS complex during ventricular pacing. Inpseudofusion, the pacing stimulus is ineffective because the tissuearound the electrode has already spontaneously depolarized and is in itsrefractory period. During a capture verification procedure, it isdesirable to detect fusion and pseudofusion beats to prevent falsecapture detection.

A method of detecting fusion/pseudofusion beats in accordance with thepresent invention relies upon canceling a template representative of acaptured response from a sensed cardiac waveform and examining theresultant waveform. The resultant captured response canceled waveformfor a fusion or pseudofusion beat will have a different waveform whencompared to the captured response template in a fusion/pseudofusiondetection window. FIGS. 13A and 13B illustrate representative waveformsof a captured response and a pseudofusion beat, respectively.

Detection of fusion/pseudofusion in accordance with a method of thepresent invention is illustrated with reference to the flow graph ofFIG. 14. A captured response template is provided by delivering pacepulses to the heart at a voltage greater than the capture threshold. Thecaptured response template is a waveform representative of the cardiacsignal resulting from a heart contraction and includes both the pacingartifact and the evoked response. Providing a captured response templatemay also encompass periodically updating the captured response template.The captured response template may be periodically updated by averaging,or otherwise combining, the cardiac signal of a captured beat with theexisting captured response template in accordance with the templateupdate methods discussed above.

FIG. 14 illustrates a method of detecting a fusion/pseudofusion beataccording to an exemplary embodiment of the invention. A capturedresponse template is provided by the method illustrated by blocks1401-1403. One or more pacing pulses are delivered at a voltage greaterthan the pacing threshold 1401 resulting in captured beats. The one ormore cardiac signals of the captured beats are sensed 1402 and averaged1403, or otherwise combined, to form the captured response template.

A cardiac signal following a pace pulse is sensed 1410. The cardiacsignal is examined in a fusion/pseudofusion detection window. Thefusion/pseudofusion detection window, for example, may begin at the endof a blanking period and extend for approximately 20 ms. The blankingperiod is a brief time interval, approximately 10 ms, following a pacepulse during which sensing is inhibited to prevent erroneous sensing ofresponse.

If the sensed cardiac signal is correlated to the captured responsetemplate 1420 within the detection window, no fusion/pseudofusion isdetected 1430. However, if the sensed cardiac signal is not correlatedto the captured response template 1420, fusion/pseudofusion is detected1440. Correlation of the sensed cardiac signal and the captured responsetemplate may be determined, for example, by calculating a correlationcoefficient using the Correlated Wave Analysis technique previouslydiscussed. If the correlation coefficient is greater than apredetermined value, for example, about 0.71, the sensed cardiac signalis considered to represent a captured response signal.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method of operating an implantable medical device for detecting afusion/pseudofusion beat, comprising: providing a captured responsetemplate, a pacing artifact template, and an evoked response reference;sensing a cardiac signal following a pacing pulse; canceling the pacingartifact template from the sensed cardiac signal to provide a pacingartifact canceled cardiac signal; comparing the pacing artifact canceledcardiac signal and the evoked response reference wherein capture isestablished if the pacing artifact canceled cardiac signal is comparableto the evoked response reference; and detecting a fusion/pseudofusionbeat by determining a correlation between the captured response templateand the sensed cardiac signal.
 2. The method of claim 1, wherein thecorrelation between the captured response template and the sensedcardiac signal is determined in a fusion/pseudofusion detection window.3. The method of claim 2, wherein the fusion/pseudofusion detectionwindow begins at an end of a blanking period and has a length of about20 ms.
 4. The method of claim 1, wherein providing the captured responsetemplate comprises combining one or more captured response waveforms toform the captured response template.
 5. The method of claim 4, whereincombining the one or more captured response waveforms to form thecaptured response template comprises combining the captured responsewaveforms using a weighted average.
 6. The method of claim 1, whereinthe evoked response reference comprises an evoked response template. 7.The method of claim 6, wherein the evoked response template is formedusing the pacing artifact template.
 8. The method of claim 7, whereinthe pacing artifact template is determined based on one or moreexponential functions.
 9. The method of claim 1, further comprisingupdating the captured response template using a captured responsesignal.
 10. An implantable medical device for detecting afusion/pseudofusion beat, comprising: pulse generator circuitryconfigured to generate pulses for cardiac stimulation; sensing circuitryconfigured to sense cardiac electrical signals following the pacingpulses; and capture detection circuitry configured to: cancel a pacingartifact template from the sensed cardiac signals to provide pacingartifact canceled cardiac signals; compare the pacing artifact canceledcardiac signals to an evoked response reference wherein capture isestablished if the pacing artifact canceled cardiac signals arecomparable to the evoked response reference; and detect afusion/pseudofusion beat by determining a correlation between thecaptured response template and the sensed cardiac signals.
 11. Thedevice of claim 10, wherein the capture detection circuitry is furtherconfigured to determine the evoked response reference based on one ormore non-captured response waveforms.
 12. The device of claim 10,wherein the capture detection circuitry is further configured togenerate the captured response template based on one or more capturedresponse waveforms.
 13. The device of claim 10, wherein the capturedetection circuitry is configured to generate the evoked responsereference using the pacing artifact template.
 14. The device of claim13, wherein the capture detection circuitry is configured to generatethe pacing artifact template based on one or more exponential functions.15. The device of claim 13, wherein the capture detection circuitry isconfigured to generate the pacing artifact template using one or morenon-captured responses.
 16. The device of claim 10, wherein the capturedetection circuitry is configured to determine a correlation coefficientbetween samples of a sensed cardiac signal and samples of the capturedresponse template.
 17. The device of claim 10, wherein the capturedetection circuitry is configured to update the captured responsetemplate using a captured response signal.
 18. An implantable system fordetecting a fusion/pseudofusion beat, comprising: a pulse generatorconfigured to deliver pacing pulses to a heart; sensing circuitryconfigured to sense cardiac signals following the pacing pulses; meansfor providing a captured response template and a pacing artifacttemplate; means for canceling the pacing artifact template from thesensed cardiac signals to provide pacing artifact canceled cardiacsignals; means for comparing the pacing artifact canceled cardiacsignals to an evoked response reference wherein capture is establishedif the pacing artifact canceled cardiac signals are comparable to theevoked response reference; and means for detecting thefusion/pseudofusion beat by determining a correlation between thecaptured response template and the sensed cardiac signals.
 19. Thesystem of claim 18, further comprising means for generating the evokedresponse reference based on one or more non-captured signals.
 20. Thesystem of claim 18, further comprising means for updating the evokedresponse reference and the captured response template.